1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device having a function of outputting a constant voltage and a function of detecting a constant voltage, and a method of regulating an output voltage thereof.
2. Description of the Related Art
An electric circuit used in electrical equipment is driven by an external power supply such as a battery. When a voltage value of the external power supply fluctuates, malfunction of the electric circuit and various abnormal phenomena may be caused, and thus, it is a typical approach to place a power management IC between the electric circuit and the external power supply, for regulating the external power supply so that a constant voltage is output or monitoring fluctuations of the power supply, to thereby promote stable operation. In particular, in a semiconductor integrated circuit such as a microcomputer or a CPU that is operated at increasingly lower voltages in recent years, the power management IC has been strongly required to output an accurate constant voltage and to accurately monitor the voltage value.
Exemplary power management ICs for outputting a constant voltage from an external power supply to an electric circuit include a step-down series regulator as illustrated in FIG. 3.
In this semiconductor integrated circuit, an external power supply voltage that is applied between a ground terminal 105 and a power supply terminal 106 is divided by a PMOS output element 104 and a voltage dividing circuit 103 including resistance elements 102. The voltage divided by the resistance elements 102 is input to a minus input terminal of an error amplifier 101, and is compared to a certain reference voltage value generated by a reference voltage circuit 100. Depending on a result of the comparison, the error amplifier 101 controls an input voltage of the PMOS output element 104 to change a source-drain resistance of the PMOS output element 104. As a result, the output terminal 107 has the function of outputting a constant output voltage that does not depend on the power supply voltage, but depends on the reference voltage value of the reference voltage circuit 100 and a resistance divided voltage ratio of the voltage dividing circuit 103. The output voltage is calculated by the following Expression (1):(output voltage)=(reference voltage value)×(resistance divided voltage ratio of voltage dividing circuit)  (1)
In regulating the output voltage, by changing a resistance value of the resistance element 102 in a method described below, the divided voltage ratio of the voltage dividing circuit 103 is changed to set the output voltage value at a desired value based on Expression (1). Therefore, the voltage dividing circuit of the semiconductor integrated circuit is required to be processed/corrected for each target output voltage.
Further, a voltage detector as illustrated in FIG. 4 that has the function of outputting a signal when the power supply voltage becomes a constant voltage is also one kind of the power management IC.
In this semiconductor integrated circuit, the power supply voltage that is input from the power supply terminal 106 is converted to a voltage divided by the voltage dividing circuit 103 that includes the resistance elements 102, and the converted voltage is compared to the reference voltage value of the reference voltage circuit 100 by a comparator 108. A voltage signal corresponding to a result of the comparison is output from the output terminal 107. With this mechanism, a voltage detector is realized that has the function of monitoring the power supply voltage and outputting, when the voltage becomes equal to or higher than, or, equal to or lower than a certain voltage, a signal for the purpose of performing appropriate processing.
Also, in the example illustrated in FIG. 4, by changing the resistance element 102, the divided voltage ratio of the voltage dividing circuit 103 is changed to set a desired voltage detection value based on Expression (1). Therefore, the voltage dividing circuit of the semiconductor integrated circuit is required to be processed/corrected for each target output voltage.
As the resistance element described above that is used for a voltage dividing circuit of a semiconductor integrated circuit, a diffused resistor that is a monocrystalline silicon semiconductor substrate implanted with impurities having a conductivity type opposite to that of the semiconductor substrate, a resistor formed of polycrystalline silicon implanted with impurities, or the like is used. In designing the voltage dividing circuit, when a plurality of such resistors is used, the resistors are set to have the same length, the same width, and the same resistivity. Then, the respective resistance elements are equally subjected to variations in shape in an etching process in which the shape is determined and to variations in impurity implantation. Therefore, even if the absolute values of the resistance elements vary, resistance ratios between the resistance elements can be maintained at a constant value.
When the resistance elements having a certain resistance value based on the same shape and the same resistivity are used in a voltage dividing circuit, various resistance values are realized through series connection and parallel connection of unit resistance elements 200 such as resistor groups 201 to 204 in FIG. 5. As described above, the unit resistance elements 200 are resistance elements having the same shape and the same resistivity, and thus, the high resistance ratios between the resistor groups each including the unit resistance element(s) can be maintained with high accuracy.
Further, fuses 301 to 304 of, for example, polycrystalline silicon, are formed in parallel with the resistor groups 201 to 204, respectively, so as to be cut by laser radiation from the outside. Depending on whether or not the fuses are cut by the laser radiation, a resistance value between a terminal 109 and a terminal 110 can be changed as necessary. Then, a voltage corresponding to a divided voltage ratio to a fixed resistor formed between the terminal 110 and a terminal 111 is output from the terminal 110.
In the voltage dividing circuit as described above that has a highly accurate resistance ratio, by cutting the polycrystalline silicon fuse(s) with a laser, a desired divided voltage ratio can be obtained with high accuracy, and products having various target output voltages can be manufactured using the same semiconductor integrated circuit.
A typical method of regulating an output voltage is as illustrated in FIG. 2.
First, an output voltage of a product completed in a semiconductor processing factory is measured as it is ((1) in FIG. 2). Then, based on a computational expression or a database prepared in advance depending on the output voltage, the polycrystalline silicon fuses formed in the voltage dividing circuit are processed with a laser to trim the output voltage ((2) in FIG. 2). Finally, the output voltage of the processed product is measured again to see whether or not the product is within specification as desired ((3) in FIG. 2). If the product is out of specification, the product is not shipped. Other than this, there is an online trimming method in which the resistors are gradually processed while the output voltage is monitored, and the processing is stopped when the output voltage reaches a desired value. The method illustrated in FIG. 2 is called an offline trimming method in contrast with the online trimming method.
Next, a reference voltage circuit that is used similarly in the circuits illustrated in FIG. 3 and FIG. 4 is described with reference to FIG. 6A and FIG. 6B.
A most basic related-art reference voltage circuit includes a depression (depletion) type NMOS transistor 402 and an enhancement type NMOS transistor 401. As illustrated in FIG. 6A, each of the transistors is formed on a P-type well region 5 in a semiconductor substrate 1, and includes a gate electrode 6, a gate oxide film 9, and an N-type source/drain region 12. The transistors are different from each other in that, as an impurity region for determining a threshold voltage that is formed under the gate oxide film 9, an N-channel impurity region 10 is formed with regard to the depression type NMOS transistor 402 while a P-channel impurity region 11 is formed with regard to the enhancement type NMOS transistor 403. Further, each of the transistors includes a drain terminal 2 and a source terminal 3 for controlling operation thereof, and a body terminal 4 for fixing a potential of the P-type well region.
By connecting in series the depression type NMOS transistor 402 and the enhancement type NMOS transistor 401 between a power supply terminal 403 and a ground terminal 404 as illustrated in FIG. 6B, outputting a constant current from the depression type NMOS transistor 402 as a current source, and inputting the current to the drain terminal 2 of the enhancement type NMOS transistor 401 as a load element, a voltage generated at the drain terminal of the enhancement type NMOS transistor 401 that is a constant voltage is output to a reference voltage output terminal 405 (see, for example, Japanese Patent Application Laid-open No. 2008-198775).
The related-art method of regulating an output voltage of a semiconductor integrated circuit device has the following problems.
With regard to the offline trimming method,
1) measurement is required to be performed twice and processing is required to be performed once, which delays completion of the product to hinder rapid shipment of the product,
2) investment in equipment for the processing and the measurement is huge,
3) reregulation of a product which turns out to be out of specification after the measurement in (3) in FIG. 2 is difficult, and thus, it is difficult to inhibit lowering of a yield, and the like.
In particular, with regard to 3) above, the offline trimming method formulates a computational expression and constructs a database for the regulation on the assumption that voltage division using resistors is ideally performed. However, in manufacturing a semiconductor integrated circuit, due to manufacture fluctuations, although not so much as an absolute value of a resistance value, a resistance divided voltage ratio also fluctuates, and the possibility of manufacturing an out-of-specification product cannot be reduced to zero.
Further, in this offline trimming method, a plurality of resistance elements and a plurality of fuses for the trim are necessary, and increase in product chip size is inevitable. As described above, when a more highly accurate resistance divided voltage ratio is quested, it is necessary to increase the size and the number of the resistance elements, and thus, as the specifications become stricter, the extent of increase in cost tends to become larger.
Further, the online trimming method has a fundamental problem in that, due to instability of the resistance value, it is difficult to obtain a highly accurate output voltage. This is because resistance elements to which a laser is radiated during laser processing generate heat, and thus, when the resistance elements depend on temperature, the resistance value changes accordingly, or, recrystallization of the resistance elements after the laser radiation or the like shifts the resistance value.